(269d) Overcoming Ammonia Synthesis Scaling Relations with Plasma-Enabled Catalysis

Ammonia synthesis from its elements is only feasible on metal catalysts at elevated temperatures and pressures. These temperature and pressure requirements are known to arise from the constraints imposed by linear scaling relations between the nitrogen activation energy and the adsorption energy of N-containing intermediates---it is not possible for catalysts to simultaneously have a low activation energy for nitrogen dissociation and a weak interaction with surface intermediates [1]. In this talk, I will consider the potential to overcome these constraints and enable high ammonia synthesis activity at low temperatures and pressure by coupling metal catalysts to a non-thermal electric discharge plasma.

We postulate that nitrogen excitations in the plasma phase decrease the energy required to dissociate the robust N-N triple bond. We develop a density-functional-theory-based microkinetic model to incorporate this effect, and parameterize the model using N2 vibrational excitations observed in a dielectric barrier discharge plasma. Two key insights emerge from this model. The first is that ammonia synthesis rates in the presence of the plasma are expected to be greatly enhanced over thermal rates for a given bulk temperature and pressure. Second, optimal catalyst materials and active sites in the presence of plasma excitation are not the same as those for thermal catalysis. The model provides guidance for optimizing catalysts for application with plasmas---ammonia synthesis rates observed in a dielectric-barrier-discharge plasma reactor are consistent with predicted enhancements and predicted changes in the optimal metal catalyst [2].

References

[1] Vojvodic, A. & Norskov, J. K. National Science Review. 2, 2015, 140–149.

[2] Mehta, P.; Barboun, P.; Herrera, F. A.; Kim, J.; Rumbach, P.; Go, D. B.; Hicks, J. C.; Schneider, W. F. Nature Catalysis, 2018, 1, 269–275.